Electrical network



1*- 15, 1941- A. w. VANCE 2,255,805

ELECTRICAL NETWORK Filed Oct. 25, 1939 Erwentor Patented Sept. 16, 1941 UNITED STATES PATENT OFFEQE ELECTRICAL NETVJOBK Arthur W. Vance, Haddonficld, N. J assignor to Radio Corporation of America, a corporation of Delaware Claims.

This invention relates to electrical networks or circuits to be operated at high frequencies, and has for its principal object the provision of an improved apparatus and method of operation for minimizing phase shift in the output voltage of such circuits during high frequency operation.

A copending application of Nils Johann Oman, Serial No. 153,574, filed July 14, 1937, and assigned to the same assignee as the present application, discloses one important application of the improved network in connection with the feedback of energy between the input and output circuits of electron discharge amplifiers. In the illustrated embodiment of the present application, as in that of the copending application, the improved network functions as the coupling between the last stages of an audio frequency amplifier and involves the provision of phase correcting elements which serve to bypass the leakage reactance of the coupling at high frequency.

The invention will be better understood from the following description considered in connection with the accompanying drawing, and its scope is indicated by the appended claims.

Figures 1 and 2 are explanatory diagrams relating to the operating characteristics of the in- I Vention,

Figure 3 is a wiring and block diagram illustrating a preferred embodiment of the invention, and

Figures 4 and 5 illustrate different modifications of a detail of the circuit of Fig. 1.

It is well known that transformers introduce large phase shifts at high frequencies, due to the action of the primary and secondary capacities and the leakage inductance between primary and secondary. Thus, 360 lag is approached at very high frequencies between the primary input current and the secondary voltage. If the leakage reactance could be shorted out, the maximum would become 90 lag. In transformers of ratios near unity, a condenser may be connected between primary and secondary points of like voltage and polarity and so short-out the leakage reactance between these points (for high frequencies) and greatly improve the phase and attenuation characteristics.

The approximate equivalent diagram of a transformer energized through a vacuum tube at high frequencies is shown in Fig. 1. This is seen to be similar to a low pass filter driven from a resistance generator. At high frequency, the filter itself approaches 180 lag in phase while, since its impedance becomes capacitive, the volttor Iii) is seen to approach lag. Thus, 360 total lag is approached and, of course, the attenuation becomes very great. This condition is very undesirable in feedback circuits where phase delays of in the whole feedback loop must be avoided if stability is to be obtained. If a condenser H is shunted across the reactor 12, as is shown by dotted lines, then at very high frequencies the diagram becomes equivalent to that of Fig. 2.

This is simply a capacity divider driven through a resistance and the maximum phase delay is 90 lag. Since capacitor Ii shunts inductance I2 and resistor I4 in Fig. 1, a parallel tuned circuit is formed in series between the input and output at some frequency. If H and M are made large compared to l2, the maximum impedance reached by their tuned circuit may be made negligible. If necessary, an additional resistance may be inserted in series with H to limit the tuned impedance, but this will permit the circuit to go to 180 lag at infinite frequency instead of 90. However, in practice, the resistance used may be made so small as to have negligible effect at any desired frequency.

A stepup transformer with one side common to the primary and secondary windings reduces at high frequencies to the equivalent of an autotransformer with two parallel-connected primary windings. For ratios near unity, the leakage inductance of an auto-transformer is much less than that of the equivalent transformer. For instance, for 2/1 ratio, the leakage of the autotransformer is one-fourth that of the transformer. Such a stepup transformer is interposed between the last two stages of the audio frequency channel illustrated by Fig. 3.

The channe1 of Fig. 3 includes an input transformer provided with a primary winding l5 and a secondary winding l6 which is shunted by a loading resistor H and capacitor l8 and connected in the input circuit of an amplifier IS. The output circuit of the amplifier i9 is coupled through successive stages 26 and 2| and a stepup transformer 22 to the input circuit of the last amplifier stage 23. The output circuit of the amplifier 23 is coupled to a load circuit 24 through a transformer 25.

It will be noted that the transformer 22 includes a primary winding 26 and a secondary 21', that substantially equipotential points of these windings are interconnected through a capacitor 28, and that a capacitor 29 is connected across age across the input to the filter (across capaci- 55 the section 31! of the winding 21 or between the winding transformer.

equal voltage point and the high voltage terminal of the secondary winding.

As previously indicated, such a transformer operated at high frequencies is the equivalent of an auto-transformer which exhibits a leakage reactance much lower than that of the usual two- If the winding section 30 is sandwiched between the windings 26 and 21, a great additional reduction in leakage inductance is realized. If the section 30 is shunted by the capacitor 29 as illustrated, this winding section is substantially bypassed at high frequencies and the phase lag of the voltage applied to the input circuit of the stage 23 cannot exceed 90.

Resonance effects in this type of network may be controlled by connecting across the winding section 30 various combinations of reactance and resistance such as (1) that illustrated by Fig. 4, in which the capacitors 3| to 33 and the resistors 35 and 36 become progressively smaller from left to right, or (2) that illustrated by Fig. 5, in which the capacitors 3'! to 40 and the resistors 4| to 43 are similarly arranged. Such arrangements, of

course, tend to reduce the negligible impedance at infinite frequency and to damp-out resonance with the inductances which they shunt.

Between the output circuit of the stage 23 and the input circuit of the stage IB is provided a degenerative feedback circuit which includes a unity, capacitance means connected between equal voltage points of said windings for shunting their leakage reactance at'high frequencies, said secondary winding having a high voltage section, and capacity means providing a high frequency path in shunt with said section of the secondary, whereby said section is substantially by-passed at high frequencies and the phase lag of output voltage from the network is less than ninety degrees. i

2. In an audio frequency amplifier, an interstage coupling network including an audio frequency stepup transformer provided with primary and secondary windings, means providing a high audio frequency path of relatively low impedance between substantially equal voltage points of said windings, and means providing a high audio frequency path in shunt relation to a high voltage section of said secondary winding between the terminal of said first named path on said winding and a terminal of the network.

3. In an electric wave transmission system, a signal circuit coupling network, a stepup coupling transformer included in said network, means afiording a high frequency path between substantially equal voltage points of the primary and secondary windings of said transformer, and

stantially unity transformation ratio, and means providing a second high audio frequency path between said tap and the high potential side of said circuit.

5. An audio frequency coupling circuit including a transformer having lower and higher impedance windings providing a ratio of transformation other than unity, means providing a high audio frequency signal conveying connection between the high potential end of the lower impedance winding and a tap on the higher impedance winding providing substantially unity transformation ratio, and means providing a second high audio frequency signal conveying connection between said tap and the high potential side of the circuit to which the higher impedance winding is connected.

ARTHUR W. VANCE. 

